The invention relates to a flowmeter having a first measuring tube and a second measuring tube which are arranged in a common housing and are connected to one another mechanically, an excitation device for exciting the measuring tubes to oscillation, a detector device for detecting oscillation parameters and an evaluating device, which determines a mass flow rate signal for each measuring tube from output signals from the detector device.
Such a flowmeter is known from WO 97/26508 A1. Using such a flowmeter, it is possible to determine the mass flow rate through each measuring tube and also the difference between or the sum of the two mass flows. Cases in which it is desirable to determine both the absolute value of the mass flow through each measuring tube and the difference between the mass flows are found, for example, in the field of medicine. In the case of purifying the blood of dialysis patients, the amount of fluid removed from the body must be monitored precisely. That amount appears in the process as a differential flow between the dialysate flow supplied and the dialysate flow removed, the latter being small in relation to the dialysate flow supplied. At the same time, it would of course also be desirable to discover the absolute amount of dialysate supplied.
Other applications are found in surface-coating technology, in which a certain amount must be held in store in a colour medium reservoir. It is therefore necessary to re-supply exactly the same amount of colour medium components as the amount of colour medium removed. For that reason, the difference in mass must be known. It would on the other hand of course also be desirable to know the absolute amount of colour medium used.
In the known flowmeter, basically two independent measuring systems operating according to the Coriolis principle are therefore used. Each measuring tube is excited to oscillation. The oscillation of the associated measuring tube is detected at a location other than at the point of excitation. The mass flow rate can then be determined from the phasing of the oscillation between the point of excitation and the measuring point or between two measuring points spaced from one another.
The oscillation is produced with respect to a housing and is also detected with respect to a housing. At the same time, the housing serves to secure the measuring tubes. Certain problems arise, however, as a result of the housing. The oscillations produced at the measuring tubes are transferred also to the housing or to a mechanical coupling between the tubes, which is provided to prevent oscillation loading at the point at which the tubes are fixed in the housing. The mechanical coupling between the tubes also forms an artificial node for the tubes when they are oscillating in opposite phase.
The mechanical coupling between the two measuring tubes is not critical provided the conditions in the two measuring tubes are identical, that is to say, when the through-flowing mass is approximately the same. That is the case in normal flowmeters, which detect only the difference between mass flows, because the two measuring tubes are then connected either in series or in parallel, see, for example, EP 0 244 692 A1. In that case, virtually no disturbances can be observed in the measurement result.
Problems arise, however, when the media flowing through the measuring tubes have different densities or different flow speeds or are subject to other different conditions. It has been shown that in such circumstances the measurement results do not reflect the true conditions with the necessary reliability.
The problem underlying the invention is to improve the measuring accuracy in cases in which identical conditions do not prevail in the measuring tubes.
The problem is solved in a flowmeter of the type mentioned at the beginning in that an amplitude detecting device is provided, which detects the amplitude of the oscillation of each measuring tube, and a correcting device is connected to the evaluating device, which correcting device has a flow input and an amplitude input for each measuring tube.
The flowmeter thus operates initially like a normal flowmeter according to the Coriolis principle. Each measuring tube is made to oscillate. A phase difference in the oscillations at various positions on each measuring tube is dependent upon the mass flow through the measuring tube. That phase difference (or other known values of measuring tubes operating according to the Coriolis principle) can be used to determine the mass flow, that is to say, the mass flowing through per unit time. The mass flow rate signal of each measuring tube is, however, subject to error. The xe2x80x9ccompositionxe2x80x9d of that error is now known. The amplitude of the oscillation of the other measuring tube and also the mass flow rate through the other measuring tube enter into this error. It is accordingly sufficient to supply those two values to the correcting device in order to form an error correction value and to correct the mass flow rate signal accordingly. Since the influence of one measuring tube on the other measuring tube and of the other measuring tube on the first measuring tube can be observed, only two further signals need be supplied to the correcting device in addition to the (error-affected) mass flow rate signals, namely the amplitudes of the two measuring tubes.
The amplitude detecting device is preferably combined with the detector device. For error correction, it is no longer even necessary to have separate sensors. All that is required is a type of signal generation, supplemented where appropriate. One is no longer obliged to determine only the phase displacement of the oscillation at various positions on a measuring tube, but it is possible to use one or more detectors to detect the amplitude as well.
Preferably the detector device has a separate detector arrangement for each measuring tube. The risk of further couplings"" becoming involved via the detector device is thus reduced. In corresponding manner, the excitation device can have a separate excitation arrangement for each measuring tube, for example, an electromagnet. The risk of reciprocal couplings is, however, somewhat smaller in the case of excitation.
Advantageously the correcting device produces for each measuring tube a product of the flow, amplitude and a coupling coefficient of the respective measuring tube and feeds that product back to the mass flow rate signal from the other measuring tube. A certain transient process is of course necessary until an error-free mass flow rate signal has been obtained. By means of the backwards coupling, however, error correction can be obtained with relatively little outlay.
In an alternative construction, for each measuring tube the correcting device adds a product of the mass flow rate signal, amplitude and a coupling coefficient of one measuring tube to the mass flow rate signal of the other measuring tube and divides the sum by a factor which comprises the amplitudes of the measuring tubes. This is a case of forwards coupling or regenerative coupling. In that construction, a mass flow rate signal that is free of coupling errors is obtained in every operating state.
The correcting device is preferably in the form of an electronic circuit. By means of the electronic circuit, the individual coupling factors can be readily reproduced and coupled with the respective amplitudes.
It is, in that case, advantageous for the circuit to have a memory for the coupling coefficients. The coupling coefficients can then be determined in advance for each flowmeter and stored. They are then permanently available for further operation.
The invention relates also to a method of determining the mass flow rate through two measuring tubes which are coupled mechanically and are excited to oscillation, a mass flow rate signal being determined from oscillation parameters of each measuring tube.
The above-mentioned problem is solved in that method in that the mass flow rate signal for each measuring tube is corrected by means of a correction value which contains a coupling coefficient and the amplitude of the oscillation of the other measuring tube.
As explained above in connection with the flowmeter, in that manner xe2x80x9cdisturbancesxe2x80x9d which are exerted by the two measuring tubes on each other as a result of the mechanical coupling of the two measuring tubes, for example, by way of the housing or. by way. of fastening elements on the housing, can be eliminated. Those disturbances are not critical only as long as identical conditions prevail in the two measuring tubes, that is to say, identical mass flows, identical densities or identical temperatures.
When conditions are different, those disturbances amplify a measuring error. Since the composition of the measuring error has now been determined, it can be eliminated again. The measuring error is dependent, firstly, upon the amplitude of the other measuring tube and, secondly, upon the mass flow rate through that measuring tube. The equipment-related disturbances can be combined in a constant coupling coefficient.
It is, in that case, preferable for the correction value to be formed by the product of the amplitude, coupling coefficient and corrected mass flow rate signal, the correction value being added with the opposite sign to the uncorrected mass flow rate signal such as a disturbance caused by the mechanical coupling. That correction method is a case of backwards coupling. The issue as to what effect the disturbance has upon the mass flow rate signal can be readily determined in advance. The backwards coupling must then operate with the opposite sign. When, for example, the disturbance has the effect of reducing the mass flow rate signal, the backwards coupling must carry out an addition.
In an alternative construction, the correction value is formed by the product of the uncorrected mass flow rate signal, amplitude and coupling coefficient of one measuring tube, which is added to the uncorrected mass flow rate signal of the other measuring tube, the sum being normalized to a value dependent upon both amplitudes. That is a case of forwards coupling or regenerative coupling. This has the advantage of delivering an error-free signal in every operating state.
Advantageously the coupling coefficients and, where appropriate, their temperature dependency are determined in advance by calibration. The coupling coefficients can be determined even during manufacture or in a subsequent step and can then be imparted permanently to the corresponding flowmeter. The coupling coefficients are basically dependent upon only mechanical influences, which do not alter during operation if the temperature remains constant. If changing temperatures are to be expected, the temperature dependency of the coupling coefficients can also be determined during calibration and that dependency can then be described mathematically, for example, by a polynomial.
The calibration is effected advantageously in that a flow passes through one measuring tube during calibration, but not through the other measuring tube. In that case it is possible to determine relatively precisely the effect of the oscillation of one measuring tube on the other.